Method of making rim-stabilized steel ingots

ABSTRACT

Rim-stabilized steel ingots are produced by teeming a rimmingtype steel into an ingot mold until the ingot is about 80-95 percent full, whereupon teeming is interrupted to allow a rimming action in the mold for a period of from 1/2 to 15 minutes. Thereafter, teeming is continued until the mold is full. After teeming is commenced following the rimming action, sufficient molten aluminum is added to the mold to produce an SK steel within the rim, all of said aluminum being added prior to completion of the steel teeming.

Waite States atent m1 Bales, Jr. et al.

[ Aug. 28, 1973 METHOD OF MAKING RIM-STABILIZED STEEL INGOTS Inventors: John W. Bales, Jr., North Huntingdon Twsp., Westmoreland County; Michael A. Orthoskl, Duquesne, both of Pa.

United States Steel Corporation, Pittsburgh, Pa.

Filed: Aug. 31, 1972 Appl. No.: 285,401

Related U. S. Application Data Continuation-impart of Ser. No. 118,498, Feb. 24,

1971, abandoned, which is a continuation-in-part of Ser. No. 49,189,.lune 23. 1970, abandoned.

Assignee:

US. Cl. 164/57, 75/124, 75/123,

75/45 int. Cl. 822d 27/18, 822d 27/20 Field 01 Search 164/55, 57, 56

References Cited UNITED STATES PATENTS 7/ 1940 Kinnear 164/133 2,389,516 11/1945 Kinnear, Jr. 164/96 3,127,642 4/1964 Zaeytydt 3,414,042 12/1968 Behrena et a]. 164/96 X 3,593,774 7/1971 Gribble et al 164/58 FOREIGN PATENTS OR APPLICATIONS 865,018 3/1971 Canada 164/55 Primary Examiner-J. Spencer Overholser Assistant Examiner-V. K. Rising Attorney-Forest C. Sexton [5 7] ABSTRACT 8 Claims, No Drawings METHOD OF MAKING RIM-STABILIZED STEEL INGOTS CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-impart of application Ser. No. 118,498, filed Feb. 24, 1971, and now abandoned, which was a continuation-in-part of application Ser. No. 49,189, filed June 23, 1970, and now abandoned.

BACKGROUND OF THE INVENTION It is well known that sheet steels are usually produced from either rimmed steel ingots of SK steel ingots, i.e., special skilled steel, killed (deoxidized) with aluminum. Rimmed steel is used in applications where surface quality is the most important requirement and little or no drawability is necessary, whereas SK steel must be used where deep drawability is essential.

Mor recently, a so-called rim-stabilized steel has been developed which incorporates the desired features of both rimmed and SK steels. That is, a rimstabilized steel ingot has a clean, good surface quality rim approaching that of a conventional rimmed steel and a pore-free core to yield good deep drawing characteristics approaching those of SK steel.

Rim-stabilized steels are presently produced by casting a rimming type steel (i.e., non-deoxidized) into an ingot mold and allowing the steel to rim for a predetermined time thereby forming a good surface quality rim. After rimming, aluminum pellets are added to the unsolidified steel in the ingot mold to stop the rimming action and produce a non-porous SK steel within the rimmed shell.

Although rim-stabilized steels do indeed fulfill a longfelt need for a sheet steel having good surface qualities as well as deep drawability, so many problems are encountered in producing ingots thereof that the steels properties are not as good as couled be hoped for. For example, the time available for adding, melting and distributing the aluminum in an already cast ingot is quite short when considering the rather large amount of aluminum that must be added, e.g., about 2 lb. per ton. Most frequently, therefore, the aluminum is not uniformly distributed within the molten portion of the steel, expecially in the lower portion thereof. This results of course in non-uniform deep drawing qualities.

Another problem encountered in producing rimstabilized steel results from the inability to produce a rim which is thick enough to allow removal of all sur face defects without exposing the non-metallic inclusions in the SK steel therebeneath. Generally, the rim is so thin that only a fast hot-scarfing operation on the rolling mill and a minimal amount of hand grinding is permitted. As a result, a rather large amount of surface defects must be processed into the final product.

Another significant problem results from the face that rim-stabilized steels always have some degree of porosity at the interface between the rim and the stabilized core. These pores apparently result from small gas bubbles formed by the rimming action just prior to stabilization of the core. That is to say, the rimming action results from the fomation of carbon monoxide gas at the liquid-rim interface. These gas bubbles adhere to the interface until they grow to sufficient size that the buoyant forces carry them to the surface. However, after the molten core is stabilized, those small bubbles formed just prior thereto cannot grow further and hence are trapped at the interface to cause the porosity. Although this porosity does adversely affect the steels drawing qualities, operators have been unable to produce rim-stabilized steels without such porosity.

Still another problem results from the practice of interrupted teeming to allow the limited rimming action. Inherent in this interruption, usually one-half minute, is the build-up of iron oxide scum on the exposed upper surface of the metal which increases with increased rimming time. When aluminum pellets are subsequently added on top of this surface scum, an excessive amount of refractory alumina is formed. Much of this alumina may become entrapped in the steel upon solidification. This alumina problem usually becomes even more aggravated because some of the aluminum pellets may not be quickly melted nor easily driven below the meniscus of the molten metal. Hence, some of the aluminum pellets may remain on the surface of the melt to be oxidized by air, thereby producing additional quantities of the troublesome alumina. It follows, therefore, that because of the excessive amount of alumina formed, the efficiency of the operation suffers, necessitating the addition of a substantially greater amount of aluminum pellets than is actually necessary to suitably deoxidize the cast steel.

SUMMARY OF THE INVENTION The invention is predicated upon our development of a new process for producing rim-stabilized steel wherein all of the above discussed problems are minimized or obviated. The process of this invention is based upon the addition of molten aluminum which may or may not be alloyed with elements such as silicon, columbium, titanium, or calcium to the ingots in a carefully controlled operating sequence, resulting in a minimum use of aluminum to produce an ingot having a good thick rim and a uniform SK steel core within the rim, without any porosity at the interface. Definitions The term molten aluminum as sued in this disclosure is intended to be understood to be molten metal consisting essentially of aluminum and as containing other metals only where the other metals are sepecified.

Accordingly, an object of this invention is to provide a new process for producing rim-stabilized steel ingots superior in quality to those produced by prior art processes.

Another object of this invention is to provide a process for producing a rim-stabilized steel ingot having a sufficiently thick rim to permit normal surface scarfing and grinding thereby assuring more complete removal of surface defects.

Another object of this invention is to provide a process for producing a rim-stabilized steel ingot having no porosity between the rim and the stabilized core.

A further object of this invention is to provide a process for producing a rim-stabilized steel ingot having a more uniform core of SK steel thereby yielding more uniform deep drawing qualities in sheet steels produced therefrom.

Still another object of this invention is to provide a process for producing a rim-stabilized steel ingot utilizing a minimum amount of aluminum and resulting in a minimum amount of alumina in the solidified ingot.

Another object of this invention is to provide a new and improved rim-stabilized steel having a thick rim, a

uniform core of SK steel and no porosity at the interface thcrebetween.

Another object of this invention is to provide a process for producing a rim-stabilized steel ingot containing hardening elements such as silicon, columbium, titanium, magnesium or calcium.

Still another object of this invention is to provide a method for producing rim-stabilized steel ingots whereby hot top ingot molds are not essential.

DESCRIPTION OF TEE PREFERRED EMBODIMENT As noted above, the crux of this inventive process for producing rim-stabilized steel ingots primarily resides in the addition of molten aluminum to the ingot mold instead of aluminum pellets or other solid forms of aluminum. In addition thereto, however, the process further requires strict adherence to a specific pouring procedure if the objectives of the invention are to be realized.

In accordance with this invention, a rim-stabilized steel ingot is produced in a hot top ingot mold in accordance with the following steps: (1) hot molten steel is teemed into the ingot mold at a normal rate (i.e., to tons per minute) until the mold is filled to the level of the lower edge of the hot top, or about 80-95 percent full if a hot top is not used; (2) teeming is stopped while the steel is allowed to rim for a period of from k to minutes and preferably 2 to 7 minutes; (3) teeming is resumed; (4) after teeming is resumed, commence pouring of molten aluminum into the ingot, preferably by introducing the aluminum into the molten steel stream; (5) addition of molten aluminum is completed prior to completion of steel teeming; and (6) teeming is continued for at least about one second until the hot top is filled. To fathom the importance of this exact pouring sequence, it must be remembered that the objective is a cleaner thick rimmed steel, with more uniform distribution of aluminum within the SK steel core, with a minimum formation of alumina, and therefore, with a minimum addition of aluminum. To achieve these objectives, the exact sequence above must be followed.

Considering each of the above steps in more detail, steps (1) and (2) are of course substantially as prac' ticed in the prior art, except that we use a longer teeming interruption time. Teeming, if commenced at normal rates of 5 to 10 tons per minute as is common to commerical practice, must be interrupted to allow some rimming action prior to the addition of aluminum which then stops the rimming action. As noted above, however, our teeming interruption time is generally greater than prior art practices where teeming interruption time is usually not allowed to exceed about onehalf minute. For this reason, therefore, we do produce a thicker rim.

We must acknowledge of course that a thicker rim is the obvious inherent result of increasing the teeming interruption time, and conceivably, therefore, prior art processes are capable of producing thicker rims merely by increasing this interruption time. By prior art practices, however, it is not possible to increase the teeming interruption time thereby producing a thicker rim without sacrificing other desired qualities of the steel. For example, increased interruption time will cause a heavy concentration of alumina in the bottom portion of the ingot. As noted previously, scum, high in iron oxide,

forms on the surface of the molten steel in the ingot, the amount of which is in direct proportion to the teeming interruption time. In prior art practices, the amount of this scum must be minimized in order to minimize alumina formation, as a result of the chemical reaction between the aluminum and scum, and its entrapment in the ingot when the aluminum is subsequently added. Hence, teeming interruption times in prior art practices are limited to about one-half minute as a practical balance between optimum rim thickness and minimum alumina formation and entrapment. In our process, however, surface scum is substantially eliminated, as will be described subsequently, and therefore, no disadvantages are experienced by increasing the teeming interruption time.

In step (3), teeming of the molten steel is commenced prior to the addition of any molten aluminum. This serves to deflect or drive the iron oxide scum into the melt thereby minimizing formation of massive gobs of surface alumina. The scum of course will be dispersed into fine particles and float to the top of the metal after teeming is complete, but by then the aluminum has already passed below the surface of the steel with a minimum of alumina formation. Whatever alumina is entrapped within the ingot is in fine particle form instead of cluster florm. Furthermore, since the aluminum is molten and preferably added with the stream of teemed steel, the tendency for the aluminum to float on the meniscus of the steel is greatly reduced, to even further minimize alumina formation.

According to steps (5) and (6), the aluminum addition to the ingot mold must be complete before the steel teeming-is complete. Obviously, this is essential so that the steel stream will be available throughout the entire aluminum addition period to deflect the surface scum and carry the molten aluminum deep into the ingot mold.

Although we do not fully understand why our process is unique in eliminating porosity at the rim-core interface, we believe this is due to a very rapid stabilizing effect resulting from the use of molten aluminum. Molten aluminum goes into solution so fast that it very quickly stops the rimming action, and is apparently available to deoxidize the carbon monoxide in the bubbles before the steel can solidify around the bubbles.

Because of the rather fast teeming rates used in commercial steel production in the United States, it is essential that the addition of the molten aluminum be completed rather quickly, i.e., in less time than it takes to fill the hot top of the ingot with steel, or the equivalent of this volume of metal if hot tops are not used. The actual amount of time available is a function of the ferrostatic head of molten steel in the ladle and the diameter of the pouring nozzle. Pouring rates are fastest at the midpoint of a heat because the ferrostatic head is still substantial and the refractory nozzle has eroded greatly. Whereas, the time for fllling a hot top section of a 16 ton ingot at the start of a heat may be 20 seconds, at the midpoint of a heat this time may be as low as 8 seconds. Therefore, by whatever means the aluminum is added, is should be capable of adding the total required aluminum within a period of less than 8 seconds, ideally 4 seconds or less if steel is to be teemed for a preferred two full seconds each before and after the aluminum addition. Obviously, if some means or procedure were employed for increasing the steel teeming time while the aluminum is added or if the repour volume is small, then of course proportionally slower aluminum additions would be acceptable.

We have found that by using molten aluminum and the above procedure, about 25 percent less aluminum need be added as compared to prior art processes. Hence, whereas prior art processes require the addition of at least about 2.0 pounds of solid aluminum per ton of steel, our process requires no more than about 1.5 pounds per ton to achieve the same chemical composition in the steel. This aluminum conservation is of course primarily due to the minimum formation of alumina, characteristic of our process.

In prior art processes, it is essential that hot top ingot molds be used in order to prevent the solid aluminum from adhering to mold walls above the molten metal surface. Molten aluminum, however, readily mixes with the steel. Therefore, although the above described procedure exemplifies the use of a hot-topped ingot mold in this process, such molds are not essential to this process.

Although there are many different methods by which molten aluminum could be added to the ingot mold, one method we have found particularly satisfactory is to utilize a centrifugal pump with an air driven motor and a long refractory pipe, to quickly pump the desired aluminum from a mobile gas fired crucible. With the molten aluminum at 1600F, 40 psi at the air motor would pump 8 pounds of aluminum per second through a 2 inch refractory pipe approximately 16 feet long. To pump 1.5 lb./ton to a 16 ton ingot would require a total of 24 pounds of aluminum. The above pump system was therefore more than adequate as the total aluminum was supplied in 3 seconds. It is of course necessary that the pump impellers, piping, etc., be suitably preheated prior to pumping to prevent aluminum from freezing thereto.

The practice of the invention as described above is based upon conventional teeming rates of from about 5 to tons per minute, as is the conventional practice in the United States. With such high teeming rates sufficient heat and turbulence is present in the mold so that virtually no rimming action will be effected during teeming. Therefore, the teeming interruption time of from to 10 minutes is essential, with optimum results effected at interruption times of 2 to 7 minutes. We should mention, however, that in European commercial practice, teeming rates of-about 2 to 3 tons per minute are most common. At such slow teeming rates, some rimming action will progress during teeming, i.e., during the latter part of the 5 to 8 minutes it takes to fill a -16 ton mold. In producing rim-stabilized steel ingots in Europe, therefore, the common practice is to effect the rimming action only during teeming, i.e., with no interruption time. The benefits of this invention can nevertheless be applied to European teeming rates, or any teeming rate, by adjusting the interruption time so as to provide a total rimming action time of from k to 15 minutes, and preferably 2 to 7 minutes. With slower teeming rates therefore, the interruption time may be shortened, or eliminated, to compensate for any rimming action that may progress euring teeming. That is to say, one feature of this invention resides in providing not a teeming interruption time but a rimming time of from $6 to 15 minutes and preferably 2 to 7 minutes.

In another embodiment of this invention, hardening alloy additives such as columbium, zirconium, silicon and titanium can be alloyed with the molten aluminum prior to admitting the molten aluminum to the ingot. Such practice is desirable in view of the fact that these hardening elements, particularly titanium and silicon, are also strong deoxidizers and cannot therefore be added prior to the rimming action. If such hardeners are added to the steel prior to teeming, they would cause the steel to be oxidized or partially deoxidized thereby preventing or slowing the rimming action. The amount of such hardening additives may be as high as 50 percent of the molten aluminum-hardener mixture.

EXAMPLES To more graphically illustrate the advantages of this invention, the following specific examples show comparative test results contrasting ingots produced in accordance with this invention with those produced in accordance with the prior art. In these tests, nine ingots were produced in accordance with prior art practice wherein 2 lb./ton of solid aluminum shot was added to the ingot. Twenty-eight ingots were produced in accordance with this invention using either 1.6 or 2.0 lb./ton of aluminum and rimming times of either one-half minute or 2.0 minutes. It is noted that none of the prior art examples were given rimming times of 2.0 minutes. This is because mill experience had already estabilished that extended rimming time beyond one-half minute is usually accompanied by an excessive concentration of aluminum in the bottom of the ingot (due to excessive alumina formation) and thus excessive bottom discards are required to meet specifications. Table I below shows the aluminum distribution achieved in these tests. TABLE I Total Heat Aluminum percent No. lngot Al Added Rimming Ingot lngot No. lb/ Form time Top Bottom ton (min.) 06R343 1 2.0 Solid shot 0.5 0.058 0.058 2 2.0 0.5 0.048 0.060 3 2.0 0.5 0.043 0.070 1 4 2.0 0.5 0.059 1 5 2.0 0.5 0.05 1 l 6 2.0 0.5 0.055 17 2.0 0.5 0.070 1 8 2.0 0.5 0.058 1 9 2.0 0.5 0.070 (Average) 0.0497 0.0612

06R343 10 2.0 Molten 0.5 0.080 0.1l0 II 2.0 Molten 0.5 0.059 0.058 (Average) 0.070 0.084

06R343 2.0 Molten 2.0 0.05 8 0.080 13 2.0 Molten 2.0 0.049 0.070 (Average) 0.0535 0.075

01R483 l 1.6 Molten 0.5 0.070 0.053 2 1 .6 Molten 0.5 0.070 0.070 3 1.6 Molten 0.5 0.060 0.057 4 1.6 Molten 0.5 0.060 0.100 5 1.6 Molten 0.5 0.057 0.070 6 1.6 Molten 0.5 0.060 0.070 7 1.6 Molten 0.5 0.060 01070 8 1.6 Molten 0.5 0.05 8 0.060 17 1.6 Molten 0.5 0.031 0.049 18 1.6 Molten 0.5 0.045 0.041 19 1.6 Molten 0.5 0.050 0.047 20 1.6 Molten 0.5 0.045 0.040 21 1.6 Molten 0.5 0.039 0.035 22 1.6 Molten 0.5 0.055 0.054 23 1.6 Molten 0.5 0.043 0.055 24 1.6 Molten 0.5 0.060 0.048 (Average) 0.0539 0.0574

01R483 9 1.6 Molten 2.0 0.070 0.057 10 1.6 Molten 2.0 0.05 8 0.070 1 l 1.6 Molten 2.0 0.060 0.058 12 1.6 Molten 2.0 0.048 0.056 13 1.6 Molten 2.0 0.070 0.058

7 14 1.6 Molten 2 0.039 0.058 15 1.6 Molten 2.0 0.058 0.041 16 1.6 Molten 2.0 0.047 0.055 (Average) 0.0563 0.0566

The test results shown in the above table readily show that the process of this invention is not only more efficient as compared to the prior art process, but provides a more uniform aluminum distribution. In addition, those ingots having a 2 minute rimming time had a rimmed zone substantially thicker than those rimmed for only one half minute. The rim was thick enough to be hot scarfed slowly during rolling in accordance with usual procedures and to be further surface conditioned in a cold state without any danger of penetrating the rim.

After rolling the ingots into sheet, representative samples were cut and the cut sections were examined. All those steels which had beenstabilized with aluminum shot were characterized by some porosity at the interface between the rim and the core, whereas those steel stabilized with molten aluminum were completelp free of such porosity. in addition, transverse deep-etch tests on a number of slabs revealed that the steels produced in accordance with this invention have a lower incidence of trapped dirt (alumina) below the slab surface than do slabs produced from solid aluminum injection techniques.

In another series of tests, ingots produced according to this invention were processed to cold-rolled 20-gage sheets and evaluated for press-forming performance in producing (a) a deep-reverse-drawn cup S-inches in diameter, and (b) a stretch formed dome using a inches in diameter hemispherical punch. For comparison, identical sheets commerically produced in accordance with three prior art practices were identically ly tested. These heats comprised rim stabilized steel stabilized with solid aluminum pellets, regular SK steel and regular DQ rimmed steel. Table 11 below shows the results of these tests. BEcause of the large number of SK and DQ rimmed heats tested, the table has been abbreviated to merely show the ranges of the results and averages rather than listing each of the 192 tests.

TABLE II Results of Press-forming Evaluation Press Performance. Depth for 10 Percent Breakage, inches Heat lngot Pos- S-inch- 10-inch- No. No. tion Diameter Diameter Drawn Stretched Cup Dome Solid'aluminum R-K Steel 03R857 16 T 6.68 3.11 M 6.56 3.12 B 6.38 3.16 03R864 12 T 5.93 3.10 M 5.73 3.11 B 5.70 3.12 0311874 10 T 6.17 3.10 M 6.08 3.12 13 5.99 3.16 Average for 3 heats 6.13 3.12 Molten-Aluminum R-K Steel 08L086 5 T 6.91 3.14 M 6.96 3.09 B 6.23 3.13 08L086 16 T 6.38 3.22 M 6.33 3.19 B 6.27 3.19 03L125 8 T 7.00 3.30 8 M 7.00 3.26 9 B 7.00 3.25 0lL213 7 T 7.00 3.24

M 7.()0 3.25 8 700 3.33 031.343 4 T 7.00 3.26 M 7.00 3.24 B 7.00 3.22 08L244 2 T 7.00 3.18 M 7.00 3.18 B 7.00 3.17 06L383 14 T 7.00 3.27 M 6.92 3.27 B 6.99 3.31 04L282 T5 7.00 3.27 M 7.00 3.27 B 7.00 3.31 05L462 9 T 700 323 M 100 3.27 B 7.00 3 32 Average for 9 heats 6.90 3.23 Regular SK Steel (15 heats) Range 6.33/7.00 3.0S/3.22 Average 6.68 3.13 Regular DO Rimmed Steel (17 heats) Range 5.38/60] 3 14/323 Average 5.67 3.19

" Because of die limitations, the maximum depth to which the cups can be drawn is 7.00 inches. The value 7.00 indicates that less than 10 percent fracture occurred at 7.00 inch depth.

We claim:

1. A process for producing a rim-stabilized steel ingot comprising teeming a rimming-type steel into an ingot mold, interrupting the teeming when said mold is from about to percent full for a time period sufficient to allow from k to 15 minutes of rimming action in the ingot mold, thereafter recommence temming the rimming-type steel into the ingot mold, and after teeming is recommenced following the rimming action, adding about A to 5 pounds of molten aluminum per ton of steel to the ingot mold at a rate sufficient to permit addition of all aluminum prior to completion of the recommenced teeming step.

2. The process according to claim 1 in which the rimming action is allowed to progress for a period of from 2 to 7 minutes.

3. The process according to claim I in which the teeming progresses at such a slow rate that a rimming.

action of from A to 15 minutes is effected during the first teeming step thereby eliminating the need for interruption of the teeming to allow for the rimming action.

6. The process according to claim 1 in which the molten aluminum is introduced into the stream of steel being introduced.

7. The process of claim 1 in which said molten alumiv num addition is commenced at least 1 full second after teeming is recommenced, and said molten aluminum addition is completed at least 1 full second before the recommenced teeming is completed.

8. The process of claim 1 in which the molten aluminum contains up to 50 percent of at least one element selected from the group consisting of columbium, zirconium, titanium and silicon. 

2. The process according to claim 1 in which the rimming action is allowed to progress for a period of from 2 to 7 minutes.
 3. The process according to claim 1 in which the teeming progresses at a rate of from 5 to 10 tons of steel per minute, and the teeming interruption time is from 1/2 to 12 minutes.
 5. The process according to claim 4 in which the teeming progresses at such a slow rate that a rimming action of from 1/2 to 15 minutes is effected during the first teeming step thereby eliminating the need for interruption of the teeming to allow for the rimming action.
 6. The process according to claim 1 in which the molten aluminum is introduced into the stream of steel being introduced.
 7. The process of claim 1 in which said molten aluminum addition is commenced at least 1 full second after teeming is recommenced, and said molten aluminum addition is completed at least 1 full second before the recommenced teeming is completed.
 8. The process of claim 1 in which the molten aluminum contains up to 50 percent of at least one element selected from the group consisting of columbium, zirconium, titanium and silicon.
 14. The process according to claim 1 in which the teeming progresses at a rate of less than 5 tons of steel per minute so that the rimming action commences during the first teeming step, and the teeming interruption time is shortened to compensate for said rimming action during the first teeming step. 